that fish is a female. Males have black spots on all individual scales. Females keep the almost solid black stripe with solid black splotching. Also Males will have a solid black stripe but there is always spotting on the scales. There is more blue tones in the fins an their bodies aren't near as yellow on it body. Males also are not as girthy. I am looking at my pair as I type.
As to when they get fiesty: my female is about 8" and the male is about the same. The male rules my 220. Pretty agressive. It wasn't like he "just got agressive". It was a slow process, and even now, he just minds his territory. I have the a breeding pair of Jags, a Red Terror, Buttikoferi, and a baby Red devil. All the fish are about 7-8" and the Dovii rule the tank. Been that way since they were all 3".